The present invention relates generally to a novel process for making solid solventless MQ resins having high bulk density comprising the steps of (1)(A) feeding at least one MQ resin dispersed in a volatile solvent into an extrusion device, (2) removing the volatile solvent to form a solid solventless MQ resin; and (3) recovering the solid solventless MQ resin, provided steps (1) to (3) are completed without the addition of a linear silicone fluid.
As used herein, the term “resin” describes a silicone composition wherein the molecular structure is arranged in a predominantly three dimensional network. Thus, the term silicone “resin” is used to distinguish the composition from linear silicone fluids.
The silicone resins described in the present invention are frequently designated “MQ” resins. MQ resins are macromolecular polymers comprised primarily of R3 SiO1/2 and SiO4/2 units (the M and Q units, respectively) wherein R is a functional or nonfunctional, substituted or unsubstituted monovalent radical. Those skilled in the art will appreciate that such resins may also include a limited number of R2 SiO2/2 and RSiO3/2 units, respectively referred to as D and T units. As used herein, the term “MQ resin” means that, on average, no more than about 20 mole percent of the resin molecules are comprised of D and T units.
Processes for making solid solventless MQ resins are known. Since MQ resins with M to Q ratios of less than 1 are typically solids at room temperature and very high viscosity materials in the molten state at elevated temperatures, a viable process must be able to handle this solid material. Therefore, the removal of the solvent from solid MQ resins in a process such as a wiped film evaporator which relies on temperature and gravity to move the material through the process as solvent is removed is not viable. Spray drying the solid MQ resin to remove solvent is a viable process (see for example U.S. Pat. No. 4,935,484, U.S. Pat. No. 5,324,806, U.S. Pat. No. 5,741,876), however, this method produces solid solventless MQ resins as a low density powder which are difficult to handle and costly to transport. Further, these MQ resins with low (less than 0.9) M to Q ratios that are best suited for spray drying into powders do not exhibit thermoplastic behavior and hence are difficult to disperse when formulating blends with other products, such as organic thermoplastics and silicone rubbers.
The inventors have found that a twin screw extruder has the torque requirements necessary to push these MQ resins having M to Q ratios less than one through the process as solvent is removed. Since a dense molten rod or sheet, or a crumbly mass exits the end of the extruder rather than a low density powder as in spray drying, the handling of the solid resin is facilitated. Simpler and less expensive materials handling, minimization of explosion hazards from dust clouds and smaller volume storage vessels are all benefits of producing high density solid resin materials rather than low density solid resin powders.
Previously, solventless silicone compositions have been prepared in an extruder by blending a MQ resin and a linear silicone fluid and then removing the solvent (U.S. Pat. No. 5,708,098). However, while these materials may have higher bulk density then powders, their utility in formulations are limited because of the presence of the linear silicone fluid which can cause migration issues in downstream formulating of release coatings, plastics additives or processing aids. Therefore, it would be highly desirable to provide a process of preparing solid solventless MQ resins where the solid solventless MQ resins have a high bulk density and are not mixed with linear silicone fluids.
The inventors have unexpectedly found that solid solventless MQ resins may be prepared without linear silicone fluid using an extrusion device by utilizing MQ silicone resins having a certain M to Q ratio. Further, the inventors have determined that cofeeding a propyl silsesquioxane (TPr) resin with the MQ resin facilitates the processing of MQ resins with low (less than 0.9) M to Q ratios.